1. Field of the Invention
The present invention relates to a power steering apparatus and, more particularly, relates to a power steering apparatus suited to generate a steering assisting force in a vehicle.
2. Description of Related Art
An apparatus generating a steering assisting force in a vehicle is disclosed, for instance, in Japanese Patent Application Laid-Open No. HEI 2-34473 (which is based on U.S. Pat. No. 4,819,545). This apparatus includes a valve shaft and a valve body. The valve shaft is concentric with the valve body and the former can rotate relative to the latter. The valve shaft and the valve body form a relative rotational angle, which will hereinafter be referred to as an operational angle.
A steering wheel is coupled to the valve shaft, while wheels to be steered are connected with the valve body via a pinion gear. The valve body and the valve shaft are coupled to each other via a torsion bar and a mechanical type reaction force mechanism. The mechanical type reaction force mechanism is composed of 1 a first V-groove ring that has a first V-shaped groove and is attached to the valve shaft, 2 a second V-groove ring that has a second V-shaped groove and is attached to the valve body, 3 a steel ball sandwiched between the V-shaped groove in the first V-groove ring and the V-shaped groove in the second V-groove ring and 4 a spring member urging the first V-groove ring toward the second V-groove ring.
The mechanical type reaction force mechanism thus constructed prevents the valve shaft and the valve body from forming an operational angle unless a torque effective therebetween exceeds a predetermined value. If an operational angle is formed between the valve shaft and the valve body, the torsion bar generates an angle of torsion equal to the operational angle.
If the torsion bar generates an angle of torsion, a steering reaction force corresponding to the angle of torsion is transmitted to the steering wheel. The aforementioned apparatus based on the related art is constructed such that a steering assisting force corresponding to an operational angle formed between the valve shaft and the valve body is transmitted to the valve body. Thus, the aforementioned conventional apparatus can 1 prevent generation of a steering assisting force unless the steering torque inputted to the steering wheel exceeds a predetermined value and 2 generate a steering assisting force corresponding to the steering torque if the steering torque exceeds the predetermined value.
Since no steering assisting force is generated until the steering torque exceeds a predetermined value, the conventional apparatus achieves a high rigidity feel in the neutral position of the steering wheel as well as enhanced steering stability. In addition, since a steering assisting force corresponding to the steering torque is generated when the steering torque exceeds the predetermined value, nimbleness during the steering operation can be achieved. Thus, the aforementioned apparatus based on the related art can provide stability and nimbleness during the steering operation.
In the aforementioned apparatus, the steel ball of the mechanical type reaction force mechanism is inhibited from being displaced solely by the first V-shaped groove in the first V-groove ring and the second V-shaped groove in the second V-groove ring. In other words, the apparatus is not equipped with any member for restricting displacement of the steel ball except for the first V-groove ring and the second V-groove ring. Thus, the apparatus may allow the steel ball to pass across the V-shaped groove in the first V-groove ring or the V-shaped groove in the second V-groove ring and slip out of its predetermined location. In this respect, the mechanical type reaction force mechanism does not provide the apparatus with the optimal structure for ensuring improved durability as well as high reliability.